Grease composition

ABSTRACT

The invention provides a grease composition which contains a base oil and a thickener, where the thickener is a diurea compound represented by formula (1), and the thickener is contained in an amount of 3 to 20 mass % in the grease composition: 
     
       
         
         
             
             
         
       
     
     wherein R 2  is diphenylmethane group; and R 1  and R 3 , which may be the same or different represent cyclohexyl group or a straight-chain or branched alkyl group having 8 to 22 carbon atoms, with the ratio of the number of moles of the cyclohexyl group to the total number of moles of the cyclohexyl group and the alkyl group, expressed as [((the number of the cyclohexyl group)/(the number of the cyclohexyl group+the number of the alkyl group))×100] being 70 to 80 mol %.

TECHNICAL FIELD

The present invention relates to a grease composition that can be used for lubricating a variety of mechanical parts such as rolling bearings, gear wheels, ball threads, linear motion bearings, cams, joints or the like.

BACKGROUND ART

From the recent perspective of reduction in energy consumption, not only the automobile, but also the electrical machinery and appliances and mechanical parts used in any industries have been required to improve the operating efficiency. In line with this, extensive studies have been made in terms of weight reduction of the parts, improvement of the structure, and the like. In particular, the mechanical parts having a rotating body are subject to energy loss due to the resistance generated by stirring the lubricant. Therefore, there has been a demand for a lubricant with reduced resistance to stirring.

By way of example, lowering the viscosities of engine oils and transmission oils is proposed to decrease the resistance to stirring of the employed lubricating oils (Tohyama, Ohmori, Tsutsui and Yamamoto, “Low Friction Gasoline Engine Oil—Effects of Lower Viscosity and Friction Modifiers-” R&D Review of Toyota CRDL, Vol. 32, No. 4 (December 1997); and Kanamoto, Ueno, Katayama and Satou, “Transmission Technology Trends and Product Developments” NTN Technical Review, No. 75 (2007)). However, the above-mentioned measures may lead to broken oil film resulting from the lowered oil viscosities, which may raise concern about damage on the surface to be lubricated.

Grease has advantages over lubricating oil in that lubrication can be realized with no supply for an extended period of time, the seal for grease can be made simpler, the amount necessary for lubrication is smaller, damages due to the leakage can be reduced, and the like. Therefore, most of the mechanical parts having rotating bodies employ the grease. On the other hand, however, the grease has higher apparent viscosity and larger resistance to stirring than the lubricating oil because the grease is a semi-solid lubricant comprising a base oil and a thickener.

To lower the resistance to stirring of the grease, it has been regarded as the usual measures to make the kinematic viscosity of base oil as low as possible, which is the same as in the case of lubricating oil. For example, JP 2009-132754 A proposes a grease composition comprising a base oil which contains a diester oil having a particular structure, and describes in Examples the use of a diester oil having a kinematic viscosity at 40° C. of 20.16 mm²/s.

When the kinematic viscosity of the base oil used in the grease is lowered, the resistance to stirring can be lowered, but the problem of broken oil film occurs as described in the case of lubricating oil, so that a sufficient life, i.e., flaking life or seizure life cannot be satisfactorily ensured. In light of the above, the base oils that can be used are limited.

SUMMARY OF INVENTION Technical Problem

An object of the invention is to provide a grease composition with lowered resistance to stirring.

Another object of the invention is to provide a mechanical part where the above-mentioned grease composition is enclosed.

Solution to Problem

The inventors of the invention have solved the above-mentioned problems by choosing a proper thickener.

Namely, the invention provides the following grease composition and mechanical part:

1. A grease composition comprising a base oil and a thickener, where the thickener is a diurea compound represented by formula (I) shown below, and the thickener is contained in an amount of 3 to 20 mass % in the grease composition:

wherein R² is diphenylmethane group; and R¹ and R³, which may be the same or different represent cyclohexyl group or a straight-chain or branched alkyl group having 8 to 22 carbon atoms, with the ratio of the number of moles of the cyclohexyl group to the total number of moles of the cyclohexyl group and the alkyl group, expressed as [((the number of the cyclohexyl group)/(the number of the cyclohexyl group+the number of the alkyl group))×100] being 70 to 80 mol %.

2. The grease composition described in the above-mentioned item 1, wherein the thickener is the diurea compound of formula (I) where the alkyl group is a straight-chain alkyl group having 16 to 20 carbon atoms.

3. The grease composition described in the above-mentioned item 1 or 2, used for a rolling bearing, gear wheel, ball thread, linear motion bearing, cam, or joint.

4. A mechanical part where the grease composition described in any one of the above-mentioned items 1 to 3 is enclosed.

Advantageous Effects of Invention

The present invention can provide a grease composition with lowered resistance to stirring. The grease composition of the invention can reduce the occurrence of broken oil film to meet a sufficiently long life, without decreasing the kinematic viscosity of the base oil.

DESCRIPTION OF EMBODIMENTS Thickener

In general, diurea compounds are widely used as the thickener for grease because diurea grease can exhibit long lubricating life and excellent versatility. A diurea grease is chosen in the invention, which is prepared by using an alicyclic amine and an aliphatic amine together at a particular molar ratio. The thus prepared diurea compound of grease is represented by the following formula (I):

In the formula (I), R² is diphenylmethane group, preferably 4,4-diphenylmethane. R¹ and R³, which may be the same or different represent cyclohexyl group, or a straight-chain or branched alkyl group, preferably a straight-chain alkyl group, having 8 to 22 carbon atoms, preferably 16 to 20 carbon atoms. In formula (I), the ratio of the number of moles of the cyclohexyl group to the total number of moles of the cyclohexyl group and the alkyl group, expressed as [((the number of cyclohexyl group)/(the number of cyclohexyl group+the number of alkyl group))×100] is 70 to 80 mol %.

The choice of the above-mentioned thickener can lessen the amount of thickener. When the above-mentioned thickener is contained in the base oil in a particular amount, the resistance to stirring of the resultant grease can be lowered. To be more specific, the amount of the thickener is in a range of 3 to 20 mass %, preferably 5 to 15 mass %, based on the total mass of the grease composition according to the invention.

Metallic soaps, Benton and silica gel cannot satisfy the requirement of heat-resistance, that is, the lubrication life of bearings at high temperatures. Fluorine-containing thickeners are considerably expensive and lacking in versatility although the heat resistance is satisfactory.

[Base Oil]

The kind and the kinematic viscosity of the base oil used in the invention are not particularly limited. It is possible to choose any kind of base oil having any kinematic viscosity as the occasion demands. When used in the rolling bearings, however, the grease may comprise a base oil preferably having a kinematic viscosity at 40° C. of 10 to 200 mm²/s, more preferably 15 to 100 mm²/s. One kind of base oil may be used alone or two or more kinds of base oils may be used in combination.

The base oils that can be used in the invention are roughly classified into mineral oils and synthetic oils. Examples of the synthetic oils include ester type synthetic oils such as diester oils and polyol ester oils, synthetic hydrocarbon oils such as poly α-olefin oils and polybutene, ether type synthetic oils such as alkyl diphenyl ethers, polyglycol type synthetic oils such as polypropylene glycols, silicone type synthetic oils, and fluorinated synthetic oils. The synthetic oils are preferred because of the excellent heat resistance. In particular, synthetic hydrocarbon oils and ester type synthetic oils are preferable. Use of such synthetic oils for the base oil can lower the resistance to stirring at low temperatures. Among the synthetic hydrocarbon oils, poly α-olefin oils are preferable. Among the ester type synthetic oils, pentaerythritol ester and trimethylolpropane ester are preferable.

[Additives]

The grease composition of the invention may further comprise any additives when necessary. Examples of the additives include an antioxidant such as amine-based and phenol-based antioxidants; an inorganic passivator such as sodium nitrite or the like; a rust inhibitor such as sulfonate-, succinic acid-, amine-, and carboxylate-based rust inhibitors; a metallic corrosion inhibitor such as benzotriazole or the like, an oiliness improver such as fatty acids, fatty acid esters, phosphates and the like; a phosphorus-containing, sulfur-containing or organic metal-containing antiwear agent or extreme-pressure agent; and a solid lubricant such as oxidized metal salts, molybdenum disulfide or the like. The contents of such additives may generally be about 0.1 to 20 mass %, preferably 0.5 to 10 mass %.

[Worked Penetration]

The worked penetration of the grease composition according to the invention, which may be adjusted according to the application of grease is preferably 200 to 440. When the grease composition is used for bearings, the worked penetration of the grease composition may preferably be in the range of 200 to 350 because too soft grease may unfavorably leak.

The grease composition of the invention can be enclosed in a variety of mechanical parts in practice. The mechanical parts include rolling bearings, gear wheels, ball threads, linear motion bearings, cams, joints or the like. More specifically, the grease composition of the invention can be used for the rolling bearings employed in a variety of motors for industrial machines, office equipment and automobiles, and in automotive electrical equipment and automotive auxiliaries; the gear wheels used for the reduction gear and the multiplying gear in windmills, robots, automobiles and the like; ball threads used in electric-powered steering, machine tools and the like; linear motion bearings, cams or joints used in the industrial machines, electronic equipment and the like.

Example 1 Preparation of Sample Greases

Grease compositions according to Examples and Comparative Examples were prepared using thickeners and base oils as shown in Table 1. In particular, 4,4-diphenylmethane diisocyanate was reacted with one or more predetermined amines in each base oil, and the reaction mixture was heated and then cooled. The resultant mixture was kneaded using a three-roll mill, whereby grease compositions according to Examples 1 and 2 and Comparative Examples 1 to 9 were obtained. A synthetic hydrocarbon oil, i.e., a poly α-olefin oil having a kinematic viscosity of 70.2 mm²/s at 40° C. when measured in accordance with JIS K 2220 23, or a diester oil, i.e., dioctyl sebacate having a kinematic viscosity of 11.6 mm²/s at 40° C. was used as the base oil.

The amount of thickener was adjusted in such a manner that the worked penetration of the resultant grease composition reached 300 (when determined in accordance with the JIS K2220 7.).

The grease compositions thus prepared were evaluated by the test methods shown below. The results are shown in Table 1.

<Test Method>

Resistance to Stirring (Rheometer)

Using a commercially available viscoelasticity measuring apparatus, Physica MCR301 made by Anton Paar GmbH, the test was carried out.

A test grease was applied between predetermined plates, which were controlled to have a predetermined gap, and the extra grease pressed out from the edge was removed. Then, the plates were rotated at a shear rate as shown below and the shear stress was read.

The test conditions are given below.

Shear rate: 1 to 10⁴s⁻¹

Temperature: 25° C.

Distance between plates: 0.2 mm

Diameter of plates: 25 mm

<Evaluation>

From the shear stress values obtained within a shear rate range of 1 to 10⁴s⁻¹ the average was calculated, which was regarded as the test result.

Less than 2000 Pa: acceptable

2000 Pa or more: unacceptable

TABLE 1 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Thick- 4,4-diphenyl-  50  50  50  50  50  50  50  50  50  50  50 ener methane (Molar diisocyanate ratio) Cyclo-  80  70  90  60  50  30  20  100 hexylamine Octa-  20  30  10  40  50  70  80 decylamine Octylamine  100 p-toluidine  100  100 Base oil Synthetic Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance hydro- carbon oil Diester oil Balance Amount of thickener   8   7  11  10  11  12  12  14  11  20  20 (mass %) Worked penetration  300  300  300  300  300  300  300  300  300  300  300 Resistance to stirring 1721 1674 2138 2092 2107 2201 2195 2688 2101 3622 2955 (Rheometer): Average of shear stress values (Pa) within shear rate range of 1 to 10⁴s⁻¹ at 25° C. Evaluation Accept- Accept- Unaccept- Unaccept- Unaccept- Unaccept- Unaccept- Unaccept- Unaccept- Unaccept- Unaccept- able able able able able able able able able able able

The grease compositions of Examples 1 and 2 where diurea thickeners specified in the invention were contained exhibited low shear stress (i.e., excellent resistance to stirring), while the grease compositions of Comparative Examples 1 to 9 not containing the diurea thickener as specified in the invention exhibited high shear stress. Comparative Example 9 showed high shear stress although a diester oil with a low viscosity was used as the base oil. The reason for this is that the thickener was not a diurea thickener according to the invention. 

1. A grease composition comprising a base oil and a thickener, where the thickener is a diurea compound represented by formula (I), and the thickener is contained in an amount of 3 to 20 mass % in the grease composition:

wherein R² is diphenylmethane group; and R¹ and R³, which may be the same or different represent cyclohexyl group or a straight-chain or branched alkyl group having 8 to 22 carbon atoms, with the ratio of the number of moles of the cyclohexyl group to the total number of moles of the cyclohexyl group and the alkyl group, expressed as [((the number of the cyclohexyl group)/(the number of the cyclohexyl group+the number of the alkyl group))×100] being 70 to 80 mol %.
 2. The grease composition of claim 1, wherein the thickener is the diurea compound of formula (I) where the alkyl group is a straight-chain alkyl group having 16 to 20 carbon atoms.
 3. The grease composition of claim 1, used for a rolling bearing, gear wheel, ball thread, linear motion bearing, cam, or joint.
 4. A mechanical part where the grease composition of claim 1 is enclosed. 